A vehicle is acted upon by a variety of forces schematically illustrated in FIG. 1. An equation describing these forces may be written asm{dot over (v)}=Fdrive−Fres−mg sin α=Fdrive−Fdrivelinelosses−Froll−Fair−mg sin α  (eq. 1)in which                m{dot over (v)} is the vehicle's weight m multiplied by the vehicle's acceleration {dot over (v)} to represent the total amount of forces acting upon the vehicle,        Fdrive is the driving force propelling the vehicle, provided mainly by the engine system,        Fres=Fdrivelinelosses+Froll+Fair is the running resistance for the vehicle,        Fdrivelinelosses is power losses in the power train, e.g. losses in the clutch, transmission and other components of the power train, and the engine system's drag losses depending on the engine system's drag torque,        Froll is the rolling resistance for the vehicle,        Fair is the air resistance for the vehicle, and        mg sin α is the force of gravity acting upon the vehicle at a road gradient α.        
The running resistance Fres is employed in many applications in a vehicle. For example, decisions by cruise controls and automatic gearchange systems are based inter alia on the magnitude of the running resistance. It is therefore very important to be able to estimate the running resistance correctly and efficiently.
Today's economical cruise controls take account inter alia of running resistance Fres. Traditional cruise controls maintain a constant reference speed vref which corresponds to the speed vset chosen by the driver. The value of the reference speed vref is only altered when the driver adjusts the set speed vset while driving. Certain economical cruise controls, e.g. Ecocruise cruise controls, have knowledge of the historical running resistance Fres,hist and try also to estimate a current running resistance Fres,pres.
An experienced driver of a vehicle without cruise control can reduce fuel consumption by adapting his/her driving to the characteristics of the road ahead so that unnecessary braking and/or fuel-consuming acceleration can be avoided. A further development of the aforesaid economical cruise controls tries to mimic the experienced driver's adaptive driving on the basis of knowledge of the road ahead so that fuel consumption can be kept as low as possible, since this very greatly affects profitability for an owner of the vehicle, e.g. a haulage company or the like.
An example of such a further development of an economical cruise control is a “look ahead” cruise control (LACC), i.e. an intelligent cruise control which uses knowledge of road sections ahead, i.e. knowledge of the characteristics of the road ahead, to determine the configuration of the reference speed vref. Here the reference speed vref is therefore allowed, within a certain speed range, to differ from the set speed vset chosen by the driver, in order to run the vehicle in a way which saves more fuel.
Knowledge of the road section ahead may for example comprise prevailing topography, road curvature and state of the road on the section ahead. This knowledge may for example be obtained from location information, e.g. GPS (global positioning system) information, map data and/or topographical map data, and weather reports. With an intelligent cruise control which takes account of the topography ahead, i.e. a look-ahead cruise control, it is therefore possible to optimise the vehicle's reference speed vref for different kinds of road, performance and train weight in order to achieve fuel saving.
An economical cruise control can, inter alia on the basis of the forces which act upon the vehicle, i.e. on the basis of the force equation (eq. 1), predict for example the vehicle's speed along a horizon of any suitable length, e.g. 1-2 km. The vehicle's future speed along the horizon may be predicted in various ways, e.g. by assuming that it will run with traditional cruise control with a reference speed vref which is the same as the set speed vset, or by assuming that the reference speed vref is allowed to vary relative to the set speed vset.
A look-ahead cruise control (LACC) does for example allow the reference speed vref to be raised, before an upgrade, to above the set speed vset, since the vehicle will presumably lose speed on the upgrade owing to high train weight relative to engine performance. Similarly, the LACC allows the reference speed vref to drop to below the set speed vset before a downgrade on which the vehicle will presumably be accelerated by its train weight. The concept here is that it is better from a fuel economy perspective to take advantage of the vehicle's acceleration by its own weight downhill than to initially accelerate before the downgrade and then brake on the downgrade. The LACC can thus reduce fuel consumption without greatly affecting journey time. Being able to determine the reference speed vref so that these fuel savings can be achieved depends on good knowledge of the vehicle's force equation (eq. 1), i.e. good knowledge of the forces which act upon the vehicle. The running resistance Fres is one of these forces.
There are also cruise controls which use a running resistance Fres as a basis for deciding how the vehicle's speed should vary. This means that such cruise controls can allow the reference speed vref to deviate from the set speed vset on the basis of at least one characteristic of the running resistance, e.g. its magnitude over time.
According to prior art described in DE102006029366, the running resistance Fres is estimated continually on the basis of the vehicle's acceleration. Such estimation may involve problems, as described in more detail below.